1. Field of the Invention
This invention broadly relates to external fixation devices for reducing bone fractures.
More particularly, the invention relates to a bone screw for use to secure external bone fracture fixation devices.
2. Prior Art
As is well known, a widely adopted method of reducing bone fractures is based nowadays on the use of so-called external fixation devices which allow fractures to be consolidated in highly critical areas, as may be especially those proximate to joints, or fractures involving serious damage to the cutaneous tissue to be treated, that is, anywhere traditional plastering may prove inappropriate or impracticable.
Such devices, usually of complex construction and supplied in varying configurations for adaptation to the most unpredictable of contingent situations, have opposite ends which are fastened to respective undamaged portions of the broken bone, using screws firmly set in the bone material of these portions.
Thus, for example in the case of a tibial fracture, the opposite ends of a corresponding (tibial) fixation device are secured across the fractured region. In other cases, where the fracture involves a joint such as an ankle, the bone screws of a corresponding external fixation device are set in the shinbone and the talus.
These bone screws for fastening the external fixation device, and thus ensuring the device effectiveness, generally include a screw head designed for engagement by a suitable driver, and a screw shank having a threaded portion which usually tapers toward a screw tip at the opposite end from said head.
In particular, the screw head is advantageously formed with a flat which extends parallel to the screw axis, milled on one side of the screw shank, for example.
In view of the overall dimensions of an external fixation device of this kind, which dimensions are largely dictated by the complexity of the device construction and functions and by its cantilever mount to the broken bone, as well as of its hindering and interfering character being a source of general xe2x80x9cinconveniencexe2x80x9d for both the surgical team and the patient, a demand exists in the art for a reduction of the device overall volume to within more acceptable limits.
A way of filling this demand could be, for example, that of using bone screws in different lengths, which would protrude only marginally from the fixation device in their installed condition.
For the purpose, the prior art provides each fixation device with a comprehensive kit of bone screws, all alike as regards design and construction but with different lengths, thereby providing, for application situations selected statistically as being those of most frequent occurrence, screws in suitable lengths which will not protrude out of the fixation device in their installed state, yet be readily accessible.
This prior approach, while being widely employed and satisfactory, is not exhaustive of all the possible situations with which a surgeon may find himself confronted, and is left with certain technical drawbacks, foremost among which is the difficulty of quickly making an unfailing assessment of the appropriate screw length in the field. It is well recognised, in fact, that despite his previous experience, skill and amount of know-how, the surgeon may not always be able to promptly gauge the most appropriate length of a screw to a particular external fixation device and a particular type of setting in a specific spot.
Consequently, in view of that once the setting operation has been started with a selected screw, it becomes most inconvenient if not impossible to discontinue the operation in order to have that screw replaced with a more suitable one, it is not infrequent for one or more bone screws to be left to protrude undesirably out of the external fixation device at the end of a setting operation, and become a likely cause of trouble.
Another drawback is the wide selection of screws that must be made available to cover the largest possible number of surgical cases. This increases the uncertainty of the choice, on the one side, and aggravates the equipment cost due to such screws.
U.S. Pat. No. 4,790,297 discloses a method and system for fixing one or more levels of the vertebrae of a spinal column. In this system cannulated wires are provided for being inserted over guide wires through rigid plates into the vertebrae for securing the plates to the vertebrae.
The heads of such screws are much thicker than their respective shanks and are divided into an upper and a lower portion by a slot. Once the screw has been fixed to the bone, this slot allows the upper portion of the head to be separated from the remainder of the screw by twisting the upper portion.
Due to the large heads and to the presence of a plate, the screws according to U.S. Pat. No. 4,790,297 have a fixed length and therefore display the same inconveniences as those of the above-cited prior art screws.
The underlying technical problem of this invention is to provide bone screws with such structural and functional features as to overcome the aforementioned drawbacks of the prior art.
The concept behind this invention is to provide a screw having a head substantially as thick as the shank with at least first and second flats, aligned to each other near the screw head and separated by a non-flat portion. The non-flat portion is advantageously formed with an annular groove.
Based on this concept, the technical problem is solved, according to this invention, by a bone screw for securing external fixation devices to broken bones which has the features set forth in claim 1 and following.
The features and advantages of a bone screw according to the invention will be apparent from the following description of an embodiment thereof, to be read in conjunction with the accompanying non-limiting drawings.